Python: skip empty and whitespace-only instructions in prepend

[Manohar Paturi (ManoharPaturi)]

Motivation & Context

prepend_instructions_to_messages adds an empty system message for each empty or whitespace-only string in the instructions list, so options like instructions=["", "Be helpful"] produce a blank system message ahead of the real one. Reported in #8523.

Description & Review Guide

• What are the major changes? Empty and whitespace-only instruction entries are skipped during preprocessing, so only instructions with content become messages.
• What is the impact of these changes? No more blank system messages from sparse instruction lists; non-empty strings behave exactly as before.
• What do you want reviewers to focus on? The skip predicate and its tests (empty string, whitespace-only, mixed lists).

Related Issue

Fixes #8523

Contribution Checklist

• The code builds clean without any errors or warnings
• All unit tests pass, and I have added new tests where possible
• The PR follows the Contribution Guidelines
• This PR is linked to an issue and there is no other open PR for this issue
• This is not a breaking change.

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[Eduard van Valkenburg (eavanvalkenburg)]

Manohar Paturi (@ManoharPaturi), thanks for the contribution. The current head is not green: Package Checks, Pre-commit Hooks, and Merge Gatekeeper are failing. The PR body also does not retain and complete the current template’s four required sections, all three Description & Review Guide prompts, a valid closing reference to #8523, or the contribution checklist. Please address the check failures and update the PR body, then re-request review.

[Manohar Paturi (ManoharPaturi)]

Eduard van Valkenburg (@eavanvalkenburg) the branch was cut from an older main which is what the failing package/pre-commit checks were tripping over. rebased onto current main (the actual change is 2 files, 28 lines), tests pass, and the body now follows the template with Fixes #8523. pushed in 9e42c3b.

[Manohar Paturi (ManoharPaturi)]

Eduard van Valkenburg (@eavanvalkenburg) the remaining pre-commit failure was two stray blank lines at the top of the test file that pyupgrade rewrites. cleaned up and pushed, checks should go green now.

[Manohar Paturi (ManoharPaturi)]

Eduard van Valkenburg (@eavanvalkenburg) found the remaining one: the new test file was missing the copyright notice header that the core lint requires (and ruff reflowed one import while i was in there). header added, all three hooks pass locally now (ruff clean, pyupgrade idempotent, tests 3/3). pushed.